Combining light beams offers many advantages to optical systems. In particular, beams can be combined to increase overall beam intensity or to construct a composite beam having components with different characteristics. One example of a composite beam is in a two-frequency interferometer that uses a heterodyne beam containing frequency components having orthogonal polarizations. These heterodyne beams can be constructed of two input beams having different frequencies and orthogonal polarizations. Handling each input beam separately before combination permits the individual polarization components to be manipulated to differentiate other characteristics of the component beams.
An optical system can use optical fibers to transmit input beams from one or more source to a beam combiner that combines the input beams into a composite beam. Successful combination of light from optical fibers requires precision control of beams emanating from the optical fibers. One traditional approach for control of the light from an optical fiber is to rigidly mount an optical fiber in a fiber holder so that the optical fiber directs light into a collimator. Light exiting the collimator is parallel and directed along a fixed axis of the collimator. This collimation of the beam from an optical fiber can be achieved with a commercially available manipulator. Such manipulators typically allow translation of the optical fiber in two directions (x and y) to align the optical fiber with the optical axis of the collimator, which may or may not be on a separate manipulator. Alternatively, fiber optic cable assemblies can be purchased with a pre-aligned-collimator termination. This provides collimated output directly from the fiber optic cable assemblies.
The collimated beams from the optical fibers are sent to beam directing optics that direct the collimated light beams into a beam combiner. Opto-mechanical beam manipulators for the beam directing optics control the paths of the light beams entering the beam combiner so that the beams are collinear when exiting the beam combiner.
FIG. 1 illustrates a traditional optical system 100 in which optical fibers 110 and 115 supply light beams from a remote source (not shown). Collimators 120 and 125 make the light exiting respective optical fibers 120 and 125 into parallel beams 130 and 135.
Beam 130 passes through a window 140 and reflects from a mirror 150 before entering a beam combiner 160. The orientation of window 140 controls refraction of beam 130 in window 140 and allows translation of beam 130 in a plane perpendicular to the propagation direction of beam 130. The orientation of mirror 150 controls the direction of beam 130 after reflection from mirror 150. Accordingly, adjustments of the orientations of window 140 and mirror 150 provide four degrees of freedom (i.e., translations along two axes and rotations about two axes) for adjustment of the path of beam 130 into beam combiner 160.
Adjustments of mirrors 145 and 155 similarly provide four degrees of freedom for control of the path of beam 135 into beam combiner 160.
In FIG. 1, beam combiner 160 is beam splitter cube. The portion of beam 130 that passes through beam combiner 160 and the portion of beam 135 that is reflected in beam combiner 160 join to form a combined beam 170. Manipulators that control the orientation of window 140 and mirrors 145, 150, and 155 adjust the paths of beams 130 and 135 so that combiner 160 joins beams into a single collinear combined beam 170.
The approach of FIG. 1 is commonly used because of the availability of good quality, opto-mechanical beam manipulators for optical elements such as mirrors and windows. The disadvantage of the approach is the relatively large number of opto-mechanical components required to precisely position beams 130 and 135 for combination. In particular, system 100 requires two fiber/collimator manipulators for collimators 120 and 125 and four beam manipulators for window 140 and mirrors 145, 150, and 155. Associated with each of these components are their inherent inaccuracies and instabilities. Further, each additional component adds to the overall volume and cost of the beam combination unit.
An alternative approach to handling beams from optical fibers uses a single-mode, fiber optic aligner to adjust and hold the position and angle of a beam from an optical fiber. U.S. Pat. No. 5,282,393 describes an example of a single-mode, fiber optic aligner. Such aligners for two or more optical fibers can produce collimated beams that are directed along the required paths. Shortcomings of these fiber optic aligners are their relatively large size, high cost, and uncertain long-term pointing stability due to the mechanical complexity of these aligners.